Page 64 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

intermediates and transition structures are more favorable than reactions that involve 43
less stable ones. The symmetry of the molecular orbitals is a particularly important
feature of many analyses of reactivity based on MO theory. The shapes of orbitals SECTION 1.2
also affect the energy of reaction processes. Orbital shapes are defined by the atomic Molecular Orbital
Theory and Methods
coefficients. The strongest interactions (bonding when the interacting orbitals have
the same phase) occur when the orbitals have high coefficients on those atoms that
undergo bond formation.
The qualitative description of reactivity in molecular orbital terms begins with
a basic understanding of the MOs of the reacting systems. At this point we have
developed a familiarity with the MOs of ethene and conjugated unsaturated systems
from the discussion of HMO theory and the construction of the ethene MOs from
atomic orbitals. To apply these ideas to new systems, we have to be able to understand
how a change in structure will affect the MOs. One approach is called perturbation
molecular orbital theory or PMO for short. 55 The system under analysis is compared
to another related system for which the MO pattern is known. In PMO theory, the MO
characteristics of the new system are deduced by analyzing how the change in structure
affects the MO pattern. The type of changes that can be handled in a qualitative
way include substitution of atoms by other elements, with the resulting change in
electronegativity, as well as changes in connectivity that alter the pattern of direct
orbital overlap. The fundamental thesis of PMO theory is that the resulting changes in
the MO energies are relatively small and can be treated as adjustments (perturbations)
on the original MO system.
Another aspect of qualitative application of MO theory is the analysis of inter-
actions of the orbitals in reacting molecules. As molecules approach one another and
reaction proceeds there is a mutual perturbation of the orbitals. This process continues
until the reaction is complete and the product (or intermediate in a multistep reaction)
is formed. The concept of frontier orbital control proposes that the most important
interactions are between a particular pair of orbitals. 56 These orbitals are the highest
filled orbital of one reactant (the HOMO) and the lowest unfilled (LUMO) orbital
of the other reactant. We concentrate attention on these two orbitals because they
are the closest in energy. A basic postulate of PMO theory is that interactions are
strongest between orbitals that are close in energy. Frontier orbital theory proposes
that these strong initial interactions guide the course of the reaction as it proceeds to
completion. A further general feature of MO theory is that only MOs of matching
symmetry can interact and lead to bond formation. Thus, analysis of a prospective
reaction path focuses attention on the relative energy and symmetry of the frontier
orbitals.
These ideas can be illustrated by looking at some simple cases. Let us consider the
fact that the double bonds of ethene and formaldehyde have quite different chemical
reactivities. Formaldehyde reacts readily with nucleophiles, whereas ethene does not.
The bond in ethene is more reactive toward electrophiles than the formaldehyde
C=O bond. We have already described the ethene MOs in Figures 1.17 and 1.18.
How do those of formaldehyde differ? In the first place, the higher atomic number of

55 C. A. Coulson and H. C. Longuet-Higgins, Proc. R. Soc. London Ser. A, 192, 16 (1947); L. Salem,
J. Am .Chem. Soc., 90, 543, 553 (1968); M. J. S. Dewar and R. C. Dougherty, The PMO Theory of
Organic Chemistry, Plenum Press, New York, 1975; G. Klopman, Chemical Reactivity and Reaction
Paths, Wiley-Interscience, New York, 1974, Chap. 4.
56
K. Fukui, Acc. Chem. Res., 4, 57 (1971); I. Fleming, Frontier Orbital and Organic Chemical Reactions,
Wiley, New York, 1976; L. Salem, Electrons in Chemical Reactions, Wiley, New York, 1982, Chap. 6.